1. Field of the Invention
This invention relates in general to an electron gun unit which has three cathodes for a color cathode ray tube wherein electron beams emitted from plural cathodes are focusesd by a single main lens and in particular it relates to an in line plural type single electron gun unit with the cathodes arranged on a straight line.
2. Description of the Prior Art
A prior art unipotential type three beam single electron gun is illustrated in FIG. 2 and comprises coaxially and sequentially arranged first to fifth grids G1 to G5 and three cathodes K.sub.R, K.sub.G and K.sub.B which are horizontally arranged and are spaced equal distance from the first grid G1 such that their cathode surfaces are parallel to each other. The first and second grids G1 and G2 are formed so as to be cup-shaped and are provided with apertures or through-holes h.sub.1 R, h.sub.1 G, h.sub.1 B, h.sub.2 R, h.sub.2 G, h.sub.2 B through which the beams pass. The third to fifth grids G3-G5 are generally tubular shaped.
A fixed voltage in the range of zero volts on a particular range is applied to the first grid G1 and a fixed voltage of approximately 0 to 1000 volts is applied to the second grid G2. A fixed voltage of about 20 to 30 K volts is applied across each of the third and fifth grids G3 and G5 and a fixed voltage in the range of 0 to 1000 volts is applied across the fourth grid G4. A subordinate electron lens Ls is formed primarily between the second grid G2 and the third grid G3 and a main electron lens L.sub.M is formed primarily between the third, fourth and fifth grids G.sub.3, G.sub.4 to G.sub.5. The electron beams B.sub.R, B.sub.G and B.sub.B respectively emanating from the cathodes K.sub.R, K.sub.G and K.sub.B pass through the through-holes h.sub.1 R, h.sub.1 G, h.sub.1 B, h.sub.2 R, h.sub.2 G, h.sub.2 B of the first and second grids G1 and G2 into the first stage lens or subordinate lens L.sub.S and are prefocused and caused to intersect at the center of the main electron lens L.sub.M. The beams diverge from this point of intersection.
A convergent means C is mounted in the path of the electron beams B.sub.R, B.sub.G and B.sub.B which have diverged from the center of the main lens L.sub.m. The convergent means C is comprised of innerdeflection electrode plates P.sub.A, P.sub.B which cause only the center beam B.sub.G of the three beams to pass therethrough and outer electrode plates Q.sub.A, Q.sub.B arranged on the outer portions of the inner deflection electron plates and parallel thereto are utilized for converging and deflecting the beams B.sub.B and B.sub.R. The voltage applied across the outer electrode plates Q.sub.A and Q.sub.B are set so as to be lower by 500 to 2000 volts than the voltage applied across the inner electrode plates P.sub.A and P.sub.B, that is, the anode voltage such that the beam B.sub.B which passes through the opening between the electrode plates P.sub.A and Q.sub.A and the beam B.sub.R which passes between the electrode plates P.sub.B and Q.sub. B are deflected and converged on the center beam B.sub.B at different ones of a number of vertically extending strips or slits of a grid AG such as a shadow mask which is arranged adjacent a phosphor screen S. Similarly to a chromatron type phosphor screen, the phosphor screen S has a set of sequentially arranged red, green and blue phosphor stripes. The shadow mask grid AG causes the respective electron beams to land on associated phosphor lines of the phosphor surface S to produce a display. FIG. 2 illustrates a horizontal and vertical deflection device D arranged at the downstream side of the convergent means C between the convergent means and the screen S so as to control and deflect the beams.
With three beam single gun electron units such as described, the cathodes K.sub.R, K.sub.G and K.sub.B are arranged such that the electron emitting surfaces of the cathodes lie in the same plane as illustrated. In this arrangement, the electron beam B.sub.G emanating from the central cathode K.sub.G and the electron beams B.sub.R and B.sub.B emanating from the two cathodes K.sub.R and K.sub.B mounted on the side of the center cathode K.sub.G the side beams B.sub.R and B.sub.B are subject to different optimum focusing conditions from each other relative to the focusing potential of the fourth or focusing electrode G4 pass through the subordinate electron lens Ls at its end such that they are offset from its optical axis and then through the center of the main electron lens Lm at a preset angle with respect to the center optical axis of the electron lens system such that the side beams B.sub.R and B.sub.B are subject to a converging action which is stronger than that of the center beam B.sub.G which passes through the center optical axis of the lens system. Thus, because of the field or image surface curvature aberration an error .DELTA. z occurs between the image forming positions center beam B.sub.G and the side beams B.sub.R and B.sub.B. This error is proportional to the square of the angle of intersection .alpha. of the side beams B.sub.R and B.sub.B with the center axis of the main electron lens L.sub.m.
FIG. 3 is an equivalent optical model which shows that when the optimum focusing occurs for the side beams B.sub.R and B.sub.B, the center beam B.sub.G will be in the underfocused state as illustrated in FIG. 3A. On the other hand, when the optimum focusing occurs for the central beam B.sub.G, the two side beams B.sub.R and B.sub.B will be in the overfocused states as illustrated in FIG. 3B. Thus, the image forming surface of the central beam B.sub.G and those of the side beams B.sub.R and B.sub.B can be caused to coincide by decreasing or weakening the strength of the lens for the two side beams. Thus, a constant difference may be provided between the optimum focusing voltage Vf1 for the side beams B.sub.R and B.sub.B and the optimum focusing voltage Vf.sub.2 for the central beam B.sub.G which is shown in the graph of FIG. 4 wherein the focusing voltage is plotted against the cathode current Ik. The voltage difference V between the optimum focusing voltage Vf1 for the side beams B.sub.R and B.sub.B and the optimum focusing voltage Vf2 for the central beam B.sub.G may differ depending upon the angle of intersection .alpha. of the side beams B.sub.R and B.sub.B with the central axis and upon the structure of the main lens Lm. However, in an electron gun used in a common type color TV receiver, the voltage difference may be of the order of 300-400 volts. In the electron gun of the type described above, the conventional practice is to apply a focusing voltage across the fourth grid G4 so that it is intermediate between the optimum focusing voltage for the central beam B.sub.G and that for the side beams B.sub.R and B.sub.B so that the central beam B.sub.G will be in a slightly underfocused state and the side beams B.sub.R and B.sub.B will be in a slightly overfocused state. This results in that optimum focusing is not simultaneously achieved with the three beams B.sub.R and B.sub.G and B.sub.B and the resolution is thus lowered.
So as to avoid such disadvantages, another type of electron gun is known and employed in which an object point P which is the cross-over point of the center beam B.sub.G is shifted toward the rear relative to the main electron lens for subjecting the center beam B.sub.G to a more intensive focusing action in a manner such that the three beams undergo an optimum focusing simultaneously.
FIG. 5 illustrates an equivalent optical model of the known system. In FIG. 5, the cross-over points in the first grid G1 and the second grid G2 of the electron gun represent the object point P corresponding to the object of the image spot in the optical lens system. Therefore, if according to the formula ##EQU1## where f represents a focal distance of the main electron lens, A the distance between the central lens plane O of the main electron lens L.sub.m and the beam cross-over point A1 and B the distance between the central lens plane O of the main electron lens Lm and the optimum focusing position B.sub.1 of the central beam B.sub.G when the cross-over point is at A.sub.1, the object point or cross-over point P of the central beam B.sub.G is shifted to a point A.sub.2 offset by .DELTA.A from the point A.sub.1, focusing of the central beam B.sub.G is optimized at a position B.sub.2 shifted by a .DELTA.B toward the main electron lens L.sub.m from the focusing position B.sub.1.
In this manner, the side beams B.sub.R and B.sub.B are subjected to a more intense convergence than the central beam B.sub.R due to the shifting of the point of passage of the side beams B.sub.R and B.sub.B through the electron lens system. Thus, by suitably selecting the parameter .DELTA.A in the above formula, the optimum focusing position and, thus, the optimum focusing voltage of the side beams B.sub.R and B.sub.B and of the central beam B.sub.G, the beams can be caused to coincide with each other.
Since the main lens L.sub.m has spherical aberration, the image forming positions are changed with variable magnitudes of the divergent angles of the respective beams B.sub.R and B.sub.B although the object point P remains constant. Thus, for the constant parameters A and B, the larger that the angle of divergent becomes, the larger the focal distance f of the main electron lens Lm becomes and hence the optimum focusing voltage VF becomes higher.
So as to utilize this principle, the side portions of an end face 11 of the first grid G1 adjacent the side cathodes K.sub.R and K.sub.B and including the through-holes h.sub.1 R, and hhd 1B are formed as inclined surfaces 11a and 11b which incline toward the main lens Lm whereas the central portion 11c of the end face 11 facing the central cathode K.sub.G and including the through-hole h.sub.1 G bulges in the opposite direction or in the inner direction. In a complementary manner, the side portions of the end face 12 of the cup-shape second grid G2 are formed as inclined surfaces 12a and 12b that are inclined similarly to the inclined surfaces 11a and 11b of the first grid G1 and the central portion 12c including the central through-hole h.sub.2 G bulges in the direction of the first grid G1. The cathodes K.sub.R, K.sub.G and K.sub.B are arranged in the first grid G1 in a manner such that the center cathode K.sub.G is mounted back of the side cathodes K.sub.R and K.sub.B with respect to the main lens Lm.
In an alternative arrangement illustrated in FIG. 7, not only are the side portions of the first and second grids G1 and G2 formed as inclined surfaces 11a, 11b, 12a and 12b but the central portion 12c of the end face of the second grid G2 including the through-hole h.sub.2 G projects in a stepped manner of preset height as illustrated. In a complementary manner, central portion 11c of the end face 11 of the first grid G1 facing the step portion of the central portion 12 is recessed towards the inner side and formed as a step with corresponding height and the cathodes K.sub.R, K.sub.G and K.sub.B which are mounted in the first grid G1 are arranged so that the central cathode K.sub.G is mounted in back of the side cathodes K.sub.R and K.sub.B relative to the main electron beam lens Lm.
In the above described arrangements, an improvement in the optimum focusing voltage difference .DELTA. Vf between the central beam B.sub.G and the side beams B.sub.R and B.sub.B occurs with some degree of coincidence of the optimum focusing positions of the three beams. However, with a color cathode ray tube that can be used over a current range for small to large currents and which is adapted as a so-called character display system for a computer terminal device, the values of the optimum focusing voltage tend to cause the beams to become dispersed especially at the larger range of the cathode current Ik. Also, non-uniform focusing voltage tends to become more pronounced at the image periphery regions where deflection arrows also exist so that red and blue color bleeding is noticed around white characters.
Thus, with the arrangement illustrated in FIGS. 6 and 7 limitations are placed on the width of the central portions 11c and 12c which are recessed or projected from the side portions or inclined surfaces 11a, 11b, 12a and 12b with result that the lens action occurs at the outlets of the beams B.sub.G, B.sub.R and B.sub.B from the second grid by voltage intrusion from the third grid G3 so that the divergent angles .theta. of the side beams B.sub.R and B.sub.B decrease thus lowering the optimum focusing voltage of the central beam B.sub.G in the higher range of the cathode current Ik.
FIG. 8 shows the relationship between the cathode current Ik and the optimum focusing voltage Vf1 for the side beams B.sub.R and B.sub.B as well as the optimum focusing voltage Vf2 for the central beam B.sub.G for the unit shown in FIG. 7. It can be observed from the diagram of FIG. 8 that the optimum focusing voltage Vf2 for the central beam B.sub.G increases for the lower range of the cathode current Ik so as to approach the optimum focusing voltage Vf1 for the side beams B.sub.R and B.sub.B when the object point or cross-over point P of the central beam B.sub.G shifts away from the main electron beam Lm. For larger currents the divergent angle .theta. of the side beams B.sub.R and B.sub.B is lowered with the result that the optimum focusing voltage Vf2 is lowered which enlarges the voltage difference .DELTA. Vf from the optimum focusing voltage Vf1 of the side beams B.sub.R and B.sub.B.